The present invention relates to a dot line printer of mechanical type, for performing dot matrix printing of characters, graphics etc in data processing applications, and in particular to an improved printer head for use in such a dot line printer.
Various types of device have been utilized for high-speed printout of characters, numerals, graphics etc, in data processing applications. The mechanical types of high-speed printers offer some advantages over non-mechanical printers, such as a capability for simultaneous generation of a number of copies while printing is taking place. The highest printing speeds available with mechanical printer are provided by the dot matrix printers, in which characters, etc, are printed as a pattern of dots, with the dots being printed by impact of suitable narrow elements in the form of thin wires or rods, on the printer paper, acting through a printer ribbon. In one form of such printers, referred to generally as a dot line printer, the narrow stylus-shaped printer elements (which will be referred to in the following as printer rods) are arrayed in line and mounted in a printer head, which is driven to shuttle horizontally from side to side, with the printer rods being positioned with their tips closely adjacent to a printer ribbon which passes close to the surface of the printer paper. As the printer head traverses the printer paper in one direction, the printer rods are actuated to impact on the paper such as to successively print sets of dots, with these dots forming parts of the characters, etc. which are to be printed. Upon completion of that traverse of the printer head, the printer paper, generally carried on a platen, is advanced vertically by one dot pitch, whereupon the printer head performs another traverse in the opposite direction, and another series of dots are printed to form the next part of each of the characters. Thus, characters, etc, can be printed in the form of matrices of dots, by cumulative traverses of the printer paper by the printer head and successive advancements of the paper. The shapes of the characters or graphics which are printed in this way are determined by electrical signals which control the actuations of the printer rods, supplied from data processing equipment, so that such dot line printers provide a very high degree of flexibility with regard to the printout content. Generally, each of the printer rods forms only a small number of the characters in a line of characters, e.g. the effective amount of horizontal traverse performed by the printer rods corresponds to the width of two, three or four characters, i.e. one or two columns in the printout.
In such dot line printers, the printer rods are mounted on individual leaf springs, which are normally held in a tensioned state by magnetic attraction exerted through a permanent magnet. When a dot is to be printed by one of the printer rods, a signal from the data processing equipment causes a pulse of current to flow in a coil corresponding to that printer rod, in a direction such as to nullify the magnetic force attracting the leaf spring of that printer rod. The restoring force of that leaf spring therefore acts to drive the printer rod rapidly outward from the printer head, to impact on the printer paper, and at about the instant of impact, the current flow in the corresponding coil is terminated, whereby the magnetic attraction acting on the leaf spring is restored. As a result of this attraction, and of the leaf spring naturally rebounding after impact on the printer paper, the leaf spring is rapidly returned to its previous position.
In order to achieve the highest possible printing speed with such a dot line printer, it is necessary to cause each printer rod to travel from its normal stationary position (i.e. the position in which it is held by magnetic bias provided from a magnetic circuit) to impact with the printer paper, as rapidly as possible, and to then to return to its stationary position, again as rapidly as possible. In addition, the momentum with which each printer rod impacts on the printer paper should be sufficiently high to enable a number of copies to be printed simultaneously, and should also be highly uniform, to ensure clearly printed dot patterns. With regard to speed of travel of the print rods to and from impact on the printer paper, the mechanical characteristics of the leaf springs are an extremely important factor. These characteristics should be optimized in relation to the gap between the printer rods and the paper, and should be highly uniform, to ensure uniformity of print quality. To achieve such optimum characteristics, the leaf springs should be manufactured from the most suitable materials, and formed into suitable shapes. However in the case of prior art types of printer heads for dot line printers, the leaf springs themselves form part of the magnetic paths which impart bias to the springs, and so must be formed of a magnetically permeable material, which does not necessarily provide optimum mechanical characteristics. Again, since the leaf springs form part of the magnetic circuit paths, the shape and cross-sectional area of each spring must be determined to some extent by magnetic path considerations, rather than by considerations of attaining optimum mechanical characteristics. Thus, such prior art types of printer heads do not permit leaf springs to be utilized which will permit the highest possible printing speeds to be attained. Another factor affecting printing speed is the manner in which magnetic flux is caused to build up and decay as a result of pulses of current applied to the coils, which momentarily counteract the magnetic bias applied to the printer rods and thereby cause printing to occur. To achieve the highest possible printing speeds, each current pulse must build up in a predetermined and uniform manner, and must be terminated at approximately the instant when the printer rod impacts on the paper, to allow the printer rod to immediately begin to return. However with prior art types of printer heads for dot line printers, the frame of the printer head itself forms part of the magnetic circuits which bias the leaf springs, i.e. is common to all of these magnetic circuits. Thus, magnetic coupling can occur between adjacent coils, through the frame, so that when a number of coils are driven simultaneously, mutual interference can occur between them. This interference affects the rate of buildup of the drive current pulses, and can cause these pulses to be terminated prematurely, i.e. before a printer rod has impacted on the printer paper. The attractive force exerted by the magnetic bias will thereupon act on the leaf spring of that printer rod, causing the printing density to be reduced. Due to this pehnomena, non-uniformity of printing can occur, with such prior art dot line printers, when a number of printer rods are actuated simultaneously.
Another consideration in the design of such printer heads is that the drive currents passed through the coils are of sufficient magnitude, that heating of the coils occurs, with the amount of such heating rapidly increasing as the printing speed is increased, i.e. as the rate at which current pulses are applied to the coils is increased. This heating results in heating of the frame, the printer head generally comprises a plurality of longitudinal members which are mutually attached and formed of different materials, bending distortion of the frame can occur, due to differences in the coefficients of linear expansion of these members. Thus, heat generated in the coils repesents a serious problem with regard to increasing the printing speed of prior art types of dot line printers.
With a printer head for a dot line printer according to the present invention, the above problems which arise with printer heads of the prior art are effectively overcome, by a novel design which offers the advantages of simplicity of construction and ease of manufacture. An important factor in this ease of manufacture lies in the fact that with a printer head for a dot line printer according to the present invention, the positions of the printer rods can be precisely adjusted, and the printer rods then fixed in position, without magnetic bias being applied to the leaf springs. In prior art types of printer head, this magnetic bias is acting on the leaf springs carrying the printer rods while such position adjustment is being performed, and since the precision of this adjustment is critical in determining the quality of printing attained, the resultant difficulty of position adjustment represents a serious disadvantage of such a prior art type of printer head for a dot line printer, from the aspect of ease of manufacture. With a printer head according to the present invention, the magnetic bias is produced by magnet blocks, each incorporating a permanent magnet, which are freely detachable from the printer head frame, so allowing position adjustment of the printer rods to be carried out without magnetic bias being applied thereto, and thereby greatly facilitating such position adjustment. In addition, with a printer head for a dot line printer according to the present invention, at least one magnetic circuit path is provided for each of the leaf springs, i.e. for each of the printer rods. Thus, the interference described above, resulting from mutual electromagnetic coupling between adjacent coils when a number of coils are driven at the same time, is virtually eliminated. This ensures uniformity of printing density, and also uniformity of printing copy capability.